Method and apparatus for electrostatic spraying



March 1962 T. o. MATHUES 3,02

METHOD AND APPARATUS FOR ELECTROSTATIC SPRAYING Filed Aug. 11, 1955 m0, 000 mm 0.0.

I N VEN TOR.

T HWMS 0. M4 Nil/[5 WAM HIS AIM/ME) i 3,fi24,l32 Patented Mar. 6, 1962 The present method and apparatus is equally efiective NETHOD AND S FOR ELECTED when compared to any of the usual electrostatic painting STATIC SPRAYING methods and equipment now on the market and 1s less Thomas O. Mathues, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Aug. 11, 1955, Ser. No. 527,723 3 Claims. (Cl. 117-93) This invention relates to an electrostatic coating method and apparatus and is particularly concerned with electrostatic painting or covering operations wherein the paint is applied at substantially atmospheric pressure to the spray head.

It is an object of the invention to provide an apparatus and method for the electrostatic deposition of paint wherein the paint is fed, by gravity, to a stationary discharge electrode member, or spray head, and is atomized from said member onto the part to be painted.

A still further object of the invention is to provide electrostatic painting equipment which does not involve moving parts and wherein the painting pattern may be controlled by controlling the shape and disposition of the spray head.

A still further object of the invention is to provide an apparatus and method for painting objects electrostatically wherein paint is supplied to a spray head at substantially atmospheric pressure and is distributed in an atomized mist therefrom, wherein the directivity of the atomized paint may be readily controlled by the shape and/ or disposition of the spray head.

Further objects and advantages of the present invention will be apparent from the following description, reference being bad to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.

In the drawings:

FIGURE 1 is a diagrammatic View of a spray head used to paint a steering wheel, which is shown for illustrative purposes.

FIGURE 2 is a fragmentary sectional view taken on line 22 of FIGURE 1.

FIGURE 3 is -a modification of the section shown in FIGURE 2.

Electrostatic paint spraying operations are well known in the art. Equipment used for this purpose generally comprises atomizing directly from air pressurized spray guns which direct paint at the objects to be painted and wherein the objects to be painted are maintained within an electrostatic field whereby'the paint is directed to the object more uniformly and economically than is possible with conventional paint spraying methods.

This type of spray painting equipment and method is described and shown in Ransburg et al. Patent 2,247,963.

More recently, electrostatic spray painting methods have been devised wherein the paint is fed to a spray head which forms one side of the circuit wherein the article to be painted is connected to the other side of the circuit. In this instance, the spray head rotates to throw ofi the paint, which is fed centrally thereof, whereby the paint, which is thrown 01f the periphery of the head, is charged and isdirected to the object to be painted. This latter method and the equipment used therewith is quite eifective in that less over-spray is experienced than in the past and, therefore, the method is more economical.

The present invention is directed to an improvement in the method and apparatus for spray painting articles electrostatically wherein an apparatus is used which includes no moving parts and wherein the paint is fed to the apparatus or spray head by gravity, whence it is atomized through the ionizing action of the electrostatic field.

expensive to make, easier to maintain and, in general, is considerably more flexible with respect to modification of the painting pattern thereof.

The apparatus is shown diagrammatically in FIGURE 1 wherein the spray head, or discharge electrode, is shown at 20. This head is merely a sheet of metal preferably bent downward to form a plateau section 22 at one end thereof which is preferably rounded as at 24. The edge of the metal, as shown in FIGURE 2, should be rounded off at the top side thereof, as by the usual stamping operation, which leaves a wire edge, shown at 26, at the bottom edge of the cut. The plate, or spray head, 20, is supported upon a dielectric support 28 which may be in the form of a standard, not shown. On the plate, or head, 20, is fixed a paint feed tube 30 which is longitudinally disposed thereon so as to open on a sloping portion 32 of the head. The tube 30 may be rubber, rubber-like material or metal and may be connected at its other end to a pressurized paint supply tank 34 which contains suitable paint, such as solvent containing paint or lacquer. The pressure on the tank 34 is maintained only sufficient to cause the paint to flow slowly from the tube 30 onto the inclined surface 32 of the spray head 20. As the paint flows outwardly onto the inclined surface 32, it moves downwardly onto the plateau portion 22 by gravity, whence it flows over the edge 26 thereof. In place of the pressurized tank 34, the tank may be elevated so that the paint will flow therefrom by gravity.

The object to be painted, shown for illustrative purposes to be a steering wheel 36, is carried by a suitable conveyor 38 which moves through the field of spray from the head 20. The head 20 is connected to the negative output of a high voltage, direct current source, preferably a source developing in the order of about 130,000 volts. This figure may vary widely and only requires a voltage sufiicient to properly ionize the paint to atomize the same. The positive output of this source is grounded and conveyor 38 is likewise grounded. In this manner, the object 36 to be painted forms one side of the electro static circuit, while the discharge electrode, or head 20, forms the other. As soon as the current is impressed upon the plate, or head, 20, the paint which is flowing slowly by gravity and dribbling over the edge thereof becomes charged and begins to atomize outwardly in a plane substantially parallel to the plane of the plateau section 22. As the object 36 passes through this mist, or spray of paint, it is thoroughly coated and, due to the charge thereon, the particles of paint tend to coat all portions of the object, even those that are not normal to the direct atomization.

By supplying the conveyor 38 at its upper end with an actuating bar 50 and positioning suitable on and OH switches 52 and 54 in proximity thereto, it is possible'to control the current to the head 20 so that the atomizing process only occurs when the object to be painted is within the field of atomization of the spray head. Thus, as the conveyor 38 moves in the direction of the arrow, its actuating bar 50 actuates switch 52 to initiate the spraying operation and, after moving through a predetermined field of spray, the bar 5! actuates switch 54 to open circuit the system. It is apparent that the conveyor track may be continuous and the intermittent operation recurrent when a number of racks are used on the conveyor. Also, suitable relays (not shown) may be provided to hold the circuits closed or open after momentary tripping of the control switches. In this manner, there is very little overspray or waste of paint. In fact, the spray head may be used in an open room without a spray booth, without noticeable odor of the paint solvent and without noticeable over-spray on surrounding objects. It is also manifest that valve means may be provided in the paint line to intermittently allow paint to fiow therein as desired. Also, paint that runs off the head during inactive periods of the spray may be collected in a drip pan (not shown), positioned beneath the head, and may be returned to the supply tank.

The paint or lacquer normally used in spray painting operations of the type herein disclosed is a solvent containing paint wherein the solvent is generally of a volatile nature. It is to be understood, however, that this is not a limitation and that other paints or coatings may be used i.e., reactive paints, that is to say paints which chemically react with the surface to be covered may also be used in the spray operations herein described. Further, two or more materials which react with one another may be used wherein the reaction occurs during the spraying operation so that a reacted product is deposited on the surface to be coated. In fact, any material that can be atomized by the electrostatic principle may be used and therefore the type of paint is not to be considered limiting to the scope of the invention.

When painting a steering wheel, for example, as shown at 36, which comprises a steel spider having a rubberlike covering thereover, it is desirable to utilize preheating of the wheel to improve the paintability thereof in a manner similar to that disclosed in Tolle et al. Patent 2,555,- 519. If the object to be painted is conductive, this preheating is not necessary to obtain a good coating. In all cases preparation of the object to be painted includes suitable cleaning of the surface thereof to enhance adherence of the paint film thereon.

It is also possible to back up the object to be painted with a conductive shield, connected to the negative output of the current source, which tends to repel the paint and cause it to deposit on the article. This procedure is useful when the article to be painted is substantially nonconductive. This shield may also be used to alter the spray pattern to some extent or a shield not electrically included in the circuit may sometimes be used to direct the spray.

The shape of the plateau section 22 determines, to a large degree, the pattern of painting. If this plateau is substantially flat and in a plane parallel to the part to be painted, the pattern is relatively long and narrow. The pattern may be varied appreciably by changing the configuration of the edge, or by tilting the spray head in various positions. Similarly, moving the object to be painted as it passes through the field may sometimes be used to advantage.

A modification of the plateau section 22 of the head is shown in FIGURE 3 wherein a groove 52 is formed adjacent the edge portion of the plateau section 22. This groove accumulates paint and directs it around the entire periphery of the outer edge 24 of the spray head whereby more paint is available over a larger area. This likewise will modify the spray pattern to some extent, making it wider and, if properly tilted, the height of the pattern may be varied. In this connection, instead of supplying the paint to the groove 52 by means of the tube 30, centrally located in the spray head, it is possible to provide two or more tubes, feeding directly into the groove 52, in

suitable positions with respect to the edge 24. In this case, as in the embodiment shown in FIGURE 1, the inclined portion may be eliminated if the plateau section 22 is slightly inclined to cause the paint to flow over the edge thereof. Such deviations come fully within the scope of my invention.

Basically, the invention here resides in the use of a spray head that involves no moving parts and wherein the paint is supplied thereto by gravity so that atomization thereof is accomplished entirely through the electrostatic principle. In this manner, highly satisfactory spraying of paint is accomplished at a reduced cost and at a saving with respect to over-spray and wastage normally present in electrostatic paint spraying operations.

While the embodiments of the present invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. An electrostatic spray head for use in electrostatic paint spray operations, consisting of; a metal spray head including a substantially horizontal plateau portion having a discharge edge and an inclined portion connected with said plateau portion and extending upwardly therefrom, supporting means for supporting said spray head so that said plateau portion is substantially level, said plateau portion having a rounded contour at its discharge edge which is spaced from said connected inclined portion, and paint supply means for supplying flowable paint to the inclined portion whence it is distributed and flowed solely by gravity onto the plateau portion where the rate of paint flow is reduced for causing the paint to accumulate and distribute over the plateau portion and to flow over an extended portion of the discharge edge.

2. The electrostatic spray head claimed in claim 2 wherein a distributing groove is formed around said discharge edge of the plateau portion for further accumulating paint and aiding in distributing the paint uniformly along said edge.

3. A method for electrostatic-ally painting an article with a fiowable paint, comprising the steps of; preparing the article for the painting operation, providing a discharge member consisting of a horizontal plateau portion having a discharge edge thereon and an inclined portion connecting with the horizontal plateau portion remote from said edge, supplying paint to the surface of the inclined portion at a point remote from said plateau portion, flowing paint down the inclined portion solely by gravity onto the plateau portion, accumulating paint on said plateau portion for causing the paint to spread outwardly over the plateau portion and to flow over said discharge edge solely by gravity, impressing a high voltage DC. current across said discharge edge and the article to be painted for ionizing the paint as it leaves said edge and for atomizing the paint into a spray pattern as determined by the shape of said edge while simultaneously positioning said article in the field of spray of said paint.

References Cited in the file of this patent UNITED STATES PATENTS 2,048,651 Norton July 21, 1936 2,680,079 Huebner June 1, 1954 2,684,656 Ransburg July 27, 1954 2,695,002 Miller 1. Nov. 23, 1954 

3. A METHOD FOR ELECTROSTATICALLY PAINTING AN ARTICLE WITH A FLOWABLE PAINT, COMPRISING THE STEPS OF; PREPARING THE ARTICLE FOR THE PAINTING OPERATION, PROVIDING A DISCHARGE MEMBER CONSISTING OF A HORIZONTAL PLATEAU PORTION HAVING A DISCHARGE EDGE THEREON AND AN INCLINED PORTION CONNECTING WITH THE HORIZONTAL PLATEAU PORTION RE MOTE FROM SAID EDGE, SUPPLYING PAINT TO THE SURFACE OF THE INCLINED PORTION AT A POINT REMOTE FROM SAID PLATEAU PORTION, FLOWING PAINT DOWN THE INCLINE PORTION SOLELY BY GRAVITY ONTO THE PLATEAU PORTION, ACCUMULATING PAINT ON SAID PLATEAU PORTION FOR CAUSING THE PAINT TO SPREAD OUTWARDLY OVER THE PLATEAU PORTION AND TO FLOW OVER SAID DISCHARGE EDGE SOLELY BY GRAVITY, IMPRESSING A HIGH VOLTAGE 